Abstract

The concept of Ecosystem Based Fisheries Management (EBFM) has been adopted by many countries as
proposed in the 2001 Reykjavik Declaration concerning marine ecosystem utilization and management.
Under a project funded by the ICAR AP Cess Fund, the construction of a trophic model of the Arabian
Sea ecosystem off Karnataka using the ECOPATH with ECOSIM (EwE) software was carried out.
ECOPATH is a trophic accounting model that is a practical way of studying the trophic interactions of all
species in an ecosystem by incorporating the energy flows between trophic levels and interactions among
trophic components. ECOSIM calculates corresponding changes in biomass of each component when the
fishing mortality of any particular group is altered. Using equilibrium simulations, where equilibrium biomass
is plotted over a range of fishing effort (F values), ECOSIM provides the facility to predict the potential
equilibrium yield for the fished group.
The project was also able to generate growth, mortality, biological, population and ECOPATH input
parameters for 55 commercially important species which are presented as ‘Species Life History Sheets’. Besides
by extensive stomach sampling (9786 fishes and shellfishes) of 56 species a comprehensive database on
diets was made and presented as diet tables.
The Karnataka model encompassed an area of 27,000 km2 and had 24 functional ecological groups. Estimates
were made of the biomass, production/biomass ratios, consumption/biomass rates, and diet compositions
for each functional group. The total system throughout represents the size of the entire system in terms of
flow and was estimated as 11,522 t/km2/year, which is comparatively high, but is consistent with tropical
marine ecosystems with high turnover. The Karnataka model had a mean trophic level of 3.04. Gross
efficiency of the fishery was estimated as 0.0016 (higher than the global average) indicating a fishery harvesting
fishes low in the food chain. Based on the gross efficiency value of the ecosystem it can be classified as an
ecosystem, which is in the process of achieving full maturity. Fully mature ecosystems have web like
connections and are less susceptible to perturbations.
The Arabian Sea ecosystem of Karnataka had a system omnivory index of 0.299 indicating the diverse
nature of feeding interactions between trophic levels. The estimates of net system production and total
primary production by total biomass ratio also substantiated the fact that the ecosystem is immature. Trophic
pyramids representing the distribution of biomass and energy flow indicate that the transfer efficiency
from primary producers was estimated as 14.3% and from detritus 9.1%.
A simulation exercise using ECOSIM with a 17% increase in effort every year for 10 years indicated the
changes in the biomass and yield of different components of the ecosystem for different fleets such as
multi-day trawl, single-day trawl, purse-seine, hook and line, gillnet and artisanal fleets. The results show
that increase in effort does not show proportional increase in total catch and value of the fishery indicating
that effort increase is not recommended for these fleets along Karnataka coast. The present ecological
model constructed and the simulations made are a preliminary exercise and with the availability of better
information and more vigorous data, the model can be improved so as to reflect the reality in a better
fashion.